1. Field of the Invention
The present invention relates to an electronic device.
2. Description of the Related Art
Servers or personal computers have been achieving remarkable progress toward high-speed high-performance operations. For semiconductor packages which serve as the hearts of computers, LSI chips are growing in size to improve their performances. Especially in servers, the LSI chip size is increasing steadily in response to demand for enhanced performance, and a requirement for heat dissipation during operations is increasing. For this reason, cooling technologies, as well as packaging technologies, have become more important. Thus, an overall structure involving a cooling system in the packaging and mounting structure is having been emphasized.
Examples of heat dissipator include a radiation fin or a heat sink, which is placed in close contact with a heat generator (such as an LSI chip). To tightly fix the heat dissipator to the heat generator, a hole is formed penetrating through both the heat dissipator and a circuit board to fasten them together using a screw.
FIG. 1 illustrates a conventional structure employing screw fastening. A semiconductor package 1020 accommodating an LSI chip 1010 mounted on an interposer 1011 and covered with a cover 1017 is mounted on the circuit board 1001 by flip chip mounting. A heat dissipator 1030 is attached to the top face of the semiconductor package 1020 by thermal grease 1021. The heat dissipator 1030 is fastened to the circuit board 1001 by screws 1025 penetrating through the circuit board 1001 and the heat dissipator 1030. This method cannot achieve uniform fastening due to variations in the number of screws 1025 and the degree of tightening. In addition, through-holes 1002 and 1003 have to be formed in advance in both the circuit board 1001 and the heat dissipator 1030 to receive the screws 1025 for fastening them together. This arrangement causes the entire structure to become large, and is disadvantageous from the viewpoint of making the device compact. Accordingly, a simple structure for achieving the equivalent cooling function is required.
The same problem arises when a heat receiving case (not illustrated) serving as the heat dissipator is fixed to the circuit board 1001 by the screws 1025 in the through-holes 1002 to hold the heat receiving case in close contact with the semiconductor package 1020. In this case, heat from the LSI chip 1010 is transferred to refrigerant via the package cover 1017 and the thermal grease 1021, and cooling efficiency is insufficient.
One method to solve the above-described problems is to embed a magnet in the base of the semiconductor package for mounting an LSI chip and bring a magnetic material provided to the heat dissipator in contact with the magnet by magnetic attractive force. Using a magnet is advantageous because the heat dissipator is attached to the heat source with a relatively simple structure and separated from the heat source in a simple manner.
Meanwhile, a packaging structure with a direct cooling system is also known. For example, a cover is placed over the top face (opposite to a ball-grid array) of the LSI chip and the periphery of the cover is sealed using a sealing material to define an enclosed space over the semiconductor. Liquid refrigerant is ejected from a nozzle into the enclosed space to directly cool the LSI chip.
However, if magnetic attraction is applied as it is to a conventional packaging structure, another problem arises, which problem is explained in conjunction with FIG. 2. In this figure, a cooling fin (or a heat dissipator) 1030 is fixed to the top face of a semiconductor package 1020 via thermal grease 1021. A magnet 1040 is placed on a substrate 1011 on which a semiconductor package 1020 is flip-chip bonded. A magnetic material 1035 provided to the cooling fin 1030 is attracted to the magnet 1040. In this arrangement, magnetic flux lines (magnetic flux) stretch out from the magnet 1040 into the surroundings. If electrical components such as interconnects, connectors or other elements exist near the magnet, these components may be adversely affected. In FIG. 2, the magnetic field from the magnet 104 influences the interconnect 1015 formed in the substrate 1011 and noise is generated in desired signals. Besides, electric current changes due to the magnetic influence and it is likely to cause error operations. The same problem arises when a water-cooled system or a cooling chamber is used as the heat dissipator.